Unknown origin of daily modulation in airborne radon at the VNS

Determine the physical origin of the observed day–night modulation in airborne 222Rn activity at the Very Near Site (VNS) of the Chooz nuclear power plant, which shows minima around 18:00 and maxima around 06:00 at levels up to approximately 40 Bq m−3, by conducting extended-duration measurements to identify the responsible environmental or operational drivers.

Background

Airborne radon (mainly 222Rn from the 238U series) was monitored in the VNS using an Alpharad Plus device. The measurement over two days revealed a clear daily modulation with a mean of 22.1 Bq m−3 and an amplitude of 11.8 Bq m−3, with minima near 6 pm and maxima near 6 am.

Although overall radon levels are relatively low for a basement due to ventilation, the cause of this periodic modulation is not identified in the current paper, motivating longer-duration monitoring to understand its origin and potential implications for background modeling.